Polymer thin film transistors have potential applications for fabricating low-cost integrated circuits for large-area/low-end electronic devices such as active matrix liquid crystal displays, electronic paper, smart cards, radio frequency identification tags, and the like. They have the added advantages of being mechanically durable and compatible with flexible substrates, thus offering the potential of fabricating structurally flexible electronic devices. Two critical requirements for these low-cost applications are sufficient charge carrier mobility and liquid processability. Higher charge carrier mobility can be achieved through enabling material design and process innovation as disclosed in Beng Ong et al., “Design of High-Performance Regioregular Polythiophenes for Organic Thin-Film Transistors” Proceedings of the IEEE, Vol. 93, No. 8., pp. 1412-1419 (August 2005), Yiliang Wu et al., U.S. Pat. No. 6,803,262, and Yiliang Wu et al., U.S. Pat. No. 6,890,868. Liquid processability of materials at room temperature or other temperatures slightly above room temperature is generally advantageous due to the lower energy requirement and the simplification in equipment.
The semiconductor layer in organic thin-film transistors (OTFTs) is preferred to be thin such as less than about 500 nm, particularly less than about 100 nm, preferably about 50 nm. A thick film may result in high off-current due to the intrinsic conductivity of the semiconductor layer. In order to achieve a thin semiconductor layer, a low concentration of the semiconductor in a proper liquid vehicle is usually used. For example, the concentration is usually less than 5 wt %, particularly less than 1 wt %. On the other hand, the low concentration of the semiconductor usually causes the composition (semiconductor and liquid) to exhibit a low viscosity, typically just a slightly higher than the viscosity of the liquid vehicle. The low viscosity causes problems during deposition, for example, coffee ring effect during printing, flying away during spin coating. These problems are more pronounced on a hydrophobic surface which is usually required for OTFTs. As a result, the semiconductor layer is not uniform, thus device performance is poor and has large variation. Therefore, there is a need to form a semiconductor composition with a high viscosity while retaining a low concentration of the semiconductor.